The invention concerns a printed valuable document with at least one authentication mark in the form of a luminescent substance based on a host lattice doped with at least one rare earth metal.
The protection of valuable documents by means of luminescent substances has been known for a long time. The use of rare earth metals in this connection has also been discussed. These have the advantage of possessing, in the infra-red spectral region, narrow-band emission lines, which are particularly characteristic, and can therefore be safely distinguished from the emissions of other substances when using measuring technology. In order to increase protection against counterfeiting still further, the rare earth metals can be incorporated together with other substances in host lattices, with the result that the excitation and/or emission spectrum of the rare earth metal is influenced in a characteristic manner. By combination with suitably absorptive substances, for example, a part of the excitation and/or emission bands of the rare earth metal can be suppressed. This influence can, however, also take the form of a xe2x80x9cdistortionxe2x80x9d, e.g. through damping of specific regions of broad-band spectra.
Likewise for reasons of protection against counterfeiting, rare earth metals with emission lines above 1500 nm are frequently used, since detection of the emission becomes more elaborate and more difficult the further into the IR spectral region the emission lines lie. For in very general terms, the principle applies that the detection sensitivity of photodetectors decreases the longer the wavelength of the radiation to be measured, since the signal-to-noise ratio becomes smaller in the same proportion; i.e. the signals to be detected are more and more difficult to identify in the noise. The luminescent substances must therefore also be present in the valuable document in a certain minimum concentration, in order to be able to produce an adequate signal strength, which can be reliably detected in the noise. In certain instances, however, these limit concentrations necessary for detection cannot be produced; for example, if the luminescent substances have their own colour, which destroys the desired colour impression when mixed with the material of the valuable document. In some cases, the attempt is also made to reduce the risk of detection by chemical analysis of the luminescent substances contained in a valuable document by the fact that the luminescent substances are applied only in very small concentrations.
In such cases, luminescent substances must be used for which the emission lines can be readily detected even in small concentrations.
The invention is therefore based on the aim of providing a valuable document with a luminescent substance producing emissions in the near IR spectral region so that, even at low concentrations, the presence of the luminescent substance can be demonstrated in the valuable document.
The fulfilment of this objective is provided by the non-dependent claims. Further developments are the subject of the dependent claims.
The valuable document according to the invention contains at least one luminescent substance based on host lattices doped with rare earth metal, such that the rear earth metal emits in the near IR spectral region, i.e. in the wavelength range of between 0.8 xcexcm and 1.1 xcexcm. This emission range has the advantage that its existence can be readily detected with a silicon (Si), gallium arsenide (GaAs), gallium-indium-arsenide (GaxInx-1As) or germanium (Ge) photodetector, since these have a relatively high response sensitivity in this wavelength region. As optically active rare earth metals, consideration may be given to the elements ytterbium, neodymium, or praseodymium, or mixtures of these elements with one or more other rare earth metals.
These rare earth metals are embedded in a host lattice, which has effective excitation bands in the visible region of the spectrum, and transfers these excitation bands to the rare earth metals. The effective excitation bands can be realised by, for example, chromium structural elements which, according to the invention, are incorporated in a garnet or perovskite structure.
In this situation, the host lattice possesses an optical window in the near infrared spectral region, and absorbs in virtually the entire visible region of the spectrum, so that all the lines in the visible spectral region of the luminescent substance are suppressed. The excitation range of the luminescent substances overlaps the radiation range of strong light sources, such as halogen lamps, flash lamps, and similar sources. As a result of this, and due to the effective energy transfer to the rare earth metals within the host lattice, it is possible to use very small quantities of substance with the valuable documents according to the invention, without the automatic detection capability being restricted. Detection by means of chemical analysis, however, is rendered extremely difficult due to the low concentration.
The absorptive constituents of the host lattice may in part be replaced by non-absorptive aluminium. The absorption, and therefore the brightness of the luminescent substance, can be controlled through the proportion of aluminium. Luminescent substances of this type can therefore also be used as additives for lighter printing inks.